1. Field of the Invention
The present invention is directed to a computer tomography apparatus of the type wherein the detector signals can also be processed to calculate a shadowgraph of the examination subject.
2. Description of the Prior Art
In a so-called third generation computer tomography apparatus, it is known to rotate the measurement unit, for portraying a specific slice of the examination subject, by 360.degree. around the system axis and to calculate the computer tomogram from the output signals of the detector array. For defining the layer that is computer-tomographically displayed, the measuring unit is interlocked in a predetermined angular position to prevent rotation and a relative displacement between the measuring unit and the patient support ensues proceeding from a starting position. The length for this relative displacement is preset. Subsequently, radiation is triggered and the output signals of the detector array are acquired by the computer during the scanning. Only when the pre-set relative displacement has been completed, which corresponds to the desired length of the shadowgraph, is the shadowgraph calculated from the intermediately stored and partially processed output signals of the detector array. The calculation of the shadowgraph thus only ensues after the patient support has reached the pre-set final position.
The pre-setting of the length of the shadowgraph constitute an additional operating event. Since the portrayal of the shadow-graph ensues only after the measurement, it is possible that either too short or too long a measurement ensues. In the former instance, the exposure of the shadowgraph must be repeated; in both instances, the patient is exposed to unnecessary radiation.
It is also known to fashion a computer tomograph of the third or fourth generation such that a shadowgraph can be simultaneously produced with the tomographic exposure as disclosed in German Patent 41 03 588.